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\begin{equation*}\frac{D}{Dt}\mathbf{L} + \left(\nabla\cdot \mathbf{v}\right) \mathbf{L} = \frac{\partial}{\partial t} \mathbf{L}+ \nabla\cdot\left(\mathbf{v} \mathbf{L}\right) = 0\end{equation*}
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